C# 可空类型的具体使用

T? myNullableNum = rangedNum/new T?() or null;

int? intClass = null; 

int? intClass = new int?()

Nullable<int> myNullableClass = new Nullable<int>();

int myInt = (int)myNullableInt;
int myInt = myNullableInt.Value;
//以上两种方式，在可空类型实例为null时会抛出异常，可以使用以下方式代替
int myInt = myNullableInt.GetValueOrDefault(); //其重载方法可以传入默认值

int? k=10;
等同于
Nullable<int> k = new Nullable<int>(10);

int m; //默认值为0
int? m; //默认值为null

int? num = null;
var k == num ?? 9;

<data_type>?<variable_name> = null;

using System;

namespace ExampleTest
{
  class Program
  {
    static void Main(string[] args)
    {
      int? objectNull = null;
      int? k = 80;
      double? db1 = new double?();
      double? db2 = 4.56;
      bool? b = new bool?();

      Console.WriteLine($"显示可空类型{objectNull},{k},{db1},{db2}");
      Console.WriteLine($"一个可空的布尔值：{b}");
      Console.ReadLine();
    }
  }
}

using System;

namespace ExampleTest
{
  class Program
  {
    static void Main(string[] args)
    {
      double? objNull = null;
      double? k = 4.85;
      double db;
      db = objNull ?? 9.82;
      Console.WriteLine($"num3的值:{db}");
      db = k ?? 6.02;
      Console.WriteLine($"num3的值:{db}");
      Console.ReadLine();
    }
  }
}

<div>
  姓名：<input id="Name" name="Name" style="width:60px" />
  性别：<select id="Kum" name="Kum">
    <option value="0">--请选择--</option>
    <option value="1">男</option>
    <option value="2">女</option>
  </select>
</div>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace WebTest.Models
{
  public class TestInDto
  {
    /// <summary>
    /// 隔离点
    /// </summary>
    public int? Kum { get; set; }

    /// <summary>
    /// 姓名
    /// </summary>
    public string Name { get; set; }
  }
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using WebTest.Models;

namespace WebTest.Controllers
{
  public class TestController : Controller
  {
    // GET: Test
    public ActionResult Index()
    {
      return View();
    }

    public ActionResult TestFunction(TestInDto testInDto)
    {
      if (!testInDto.Kum.HasValue)
        return Json(new { Success = false, Message = "请选择性别", Data = "" });

      int? num = null;
      DateTime? dt = null;
      bool? b = null;
      var data = new TestOutDto
      {
        Name = testInDto.Name,
        Num = num ?? 10,
        Dt = dt ?? DateTime.Now,
        IsExist = b ?? false
      };
      return Json(new { Success = true, Message = "", Data = data });
    }
  }
}